1. Field of the Invention
The present invention relates to a structure for holding a bobbin around which a bobbin thread is wound within a bobbin case holder of a horizontal axis full rotary hook.
2. Description of the Related Art
FIG. 1 is an exploded perspective view showing an overall structure of a typical horizontal axis full rotary hook 22 according to a conventional technique, and FIG. 2 is a partial horizontal cross sectional view showing a bobbin 2 as it is held in a bobbin case holder 3 by means of a bobbin case 1. The bobbin case holder 3 is housed in a hook body 4, which is driven to rotate about a horizontal rotation axis line L, in such a manner that the bobbin case holder is prevented from rotating. The bobbin case 1 is detachably attached to the bobbin case holder 3, and the bobbin case 1 houses the bobbin 2 around which a bobbin thread 5 is wound.
The bobbin case 1 comprises a peripheral wall 6 which is shaped approximately in the form of a letter "C" to open at the top in FIG. 1, an end wall 7 which closes one end portion of the peripheral wall 6 in the direction of the axial line, a latch 8 which is attached to the end wall 7 for free sliding in an engagement direction A1 which is along a diametral line and in a release direction A2 which is opposite to the engagement direction A1, and a manipulation flap 9 which is disposed to displace the latch 8 in the direction of the diametral line. The latch 8 is pressed by spring force by a spring not shown in the engagement direction A1. A bobbin thread tension spring 10 is disposed to the peripheral wall 6 of the bobbin case 1. As indicated by an imaginary line 5a, the bobbin thread 5 of the bobbin 2 which is housed in the bobbin case 1 is led out through a gap between the peripheral wall 6 and the bobbin thread tension spring 10 so that it is possible to apply tension force to the bobbin thread 5.
The bobbin 2 which is housed in the bobbin case 1 comprises a pair of flanges 11a, 11b, and a winding drum 12 which is shaped as a right circular cylinder and which links the flanges 11a, 11b. The bobbin case holder 3 includes a peripheral wall 13, a bottom portion 14 which extends in the direction of the diametral line at an end portion of the peripheral wall 13 taken in an axial line direction, and a stud 15 which is disposed to the bottom portion 14 upright toward an open end side. A ring-shaped engagement recess 16 is formed in an idle end portion of the stud 15.
A cylinder portion 17 which is shaped as a right circular cylinder is formed to the end wall 7 of the bobbin case 1, to extend along an axial line of the peripheral wall 6 toward the open end side of the peripheral wall 6. The stud 15 is inserted as it is mounted to the bobbin case holder 3 in the cylinder portion 17, so that the stud 15 projects outside from a central hole 18 which is formed in the end wall 7, passes through a first through hole 19 which is formed in the latch 8, and is exposed outside as the bobbin case 1 is partially received by a second through hole 20 which is formed in the manipulation flap 9.
Thus, when the bobbin case 1 is mounted to the bobbin case holder 3, a peripheral portion 21 which is faced with the first through hole 19 of the latch 8 is engaged with the engagement recess 16 of the stud 15, whereby the bobbin case holder 3 and the bobbin case 1 engage with each other and the bobbin case 1 is locked from falling by the bobbin case holder 3. The bobbin 2 is housed in the bobbin case 1, with the cylinder portion 17 of the bobbin case 1 passing through the winding drum 12 of the bobbin 2. As described earlier, the bobbin thread 5 of the bobbin 2 is led out through the gap between the bobbin thread tension spring 10 and the peripheral wall 6 as indicated by the imaginary line 5a, so that it is possible to sew with predetermined tension force.
In the conventional technique as described above, as described earlier, when the bobbin 2 is held within the bobbin case holder 3 by means of the bobbin case 1, the stud 15 of the bobbin case holder 3 is inserted in the cylinder portion 17 of the bobbin case 1, the cylinder portion 17 is inserted in the winding drum 12 of the bobbin 2 and the bobbin 2 is supported for free rotation about a rotation axis line which is approximately coaxial with the axial line of the cylinder portion 17. In this condition, since a small gap T of about 0.2 to 0.5 mm which allows the bobbin 2 to rotate is created between the flange 11a, which is disposed on the bottom portion 14 side of the bobbin case holder 3 of the bobbin 2 (i.e., the left-hand side in FIG. 2), and the bottom portion 14 or between the other flange 11b, which is disposed on the open end 21 side of the bobbin case holder 3 of the bobbin 2 (i.e., the right-hand side in FIG. 2), and the end wall 7 of the bobbin case 1, during sewing, due to tension force of the bobbin thread 5 which is led outside from the bobbin case 1, the bobbin 2 is displaced in the range of the gap T in the axial line direction while rotating. The displacement causes the tension force of the bobbin thread 5 to vary. As a result, the thread is fastened nonuniformly, whereby the quality of sewing is deteriorated.
Further, with respect to the conventional rotary hook 22 as above, an increase in the amount of the bobbin thread which is wound up around the winding drum 12 of the bobbin 2 may be realized by modifying the structure, e.g., by extending the length H of the winding opening of the bobbin 2 toward the open end of the bobbin case holder 3 or the bottom portion 14 and reducing the diameter D of the winding drum 12. However, when the bobbin 2 is extended toward the open end of the bobbin case holder 3 and the length H of the winding drum 12 of the bobbin 2 is increased, in a path in which a needle 23 moves, the tip of the needle hits a flange 11b or the bobbin thread 5 which is wound around the winding drum 12.
Still further, when the bottom portion 14 of the bobbin case holder 3 is extended toward the hook body 4 and the length H of the winding drum 12 of the bobbin 2 is extended toward the bottom portion 14 of the bobbin case holder 3, the length of the bobbin case holder 3 taken in the axial direction becomes long. In such a structure, a needle thread gets caught by the bottom portion 14 of the bobbin case holder 3, and therefore, the thread can not get through.
On the other hand, when the diameter D of the winding drum 12 is reduced, since the cylinder portion 17 of the bobbin case 1 and the stud 15 of the bobbin case holder 3 are inserted within the winding drum 12, since the diameter D of the winding drum 12 is restricted, it is difficult to reduce the diameter D of the winding drum 12.